This invention relates to the field of multi-channeled fiber optic temperature measuring instruments and to the use of fiber optic coupled black body transducers in the wide range, rapidly changing measurement of temperatures.
High temperature fiber optic thermometry offers a plausible solution for several complex problems in the temperature measurement art. Among these problems is the measurement of rapidly fluctuating temperatures, as found in a flame; temperature measurement in the presence of strong electromagnetic fields, as in electric furnaces or induction furnaces; temperature measurement in a nuclear radiation environment, and temperature measurement with greater accuracy than afforded by the present-day thermocouple measurement. Fiber optic thermometry also provides new answers for the problems of temperature probe or thermocouple lead heat conduction, temperature probe gas flow disruption, and new approaches to laboratory standard temperature measurements. Fiber optic thermometry is perhaps most importantly, however, one of the most promosing approaches for measuring the temperature transients encountered in a turbulent combustion or sooty combustion atmosphere.
The recent advent of improved solid state phototransducer devices having useful properties in the infrared spectral range and intended principally for use in the fiber optic communications field has added new dimensions to the capability of fiber optic temperature measurement systems. An example of such phototransducer devices is found in the indium gallium arsenide photodiode device currently manufactured by RCA Corporation, and known as the C30980E Photodiode. The C30980E photodiode, which is preferred in the described embodiment of the present invention, is described technically in a data sheet titled "C30979E, C30979EQC, C30980E, C30980EL InGaA.sub.s Photodiodes" which was printed in September 1982 and July 1984 by RCA Corporation and in RCA advertisements such as appear at page 178 of the journal Laser Focus and Fiberoptic Technology, April 1982. Such photodiodes are sold by RCA Corporation from a New Products Division Office on New Holland Avenue in Lancaster, PA 17604-3140 and from a Photodetector Marketing Office located at Ste. Anne de Bellevue, Quebec, Canada H9X 3L3. The RCA and similar solid state photoelectric transducer devices which may be or become available from other commercial suppliers have favorable response characteristics in a portion of the infrared spectrum that is desirable for performing temperature measurements.
Improvements in optical temperature measurement have also been recently achieved at the National Bureau of Standards, by R. R. Dils and others, as is exemplified by the Dils articles "High-temperature Optical Thermometer" published in the Journal of Applied Physics, Vol. 54, 1983, p. 1198 and "A Fiberoptic Probe for Measuring High Frequency Temperature Fluctuations in Combustion Gases", Sandia Report 83-8871 published during the first half of 1984 by R. R. Dils and D. A. Tichenor. Both of these Dils publications are hereby incorporated herein by reference.
The patent art includes several examples of high temperature measuring systems which precede the present invention. Included in this patent art is the patent of D. A. Kahn, U.S. Pat. No. 4,326,798, which concerns an optical pyrometer system employing spectral segregation of signal components received from a workpiece being measured, i.e., from a temperature elevated engine turbine blade. The Kahn patent is especially concerned with the avoidance of measurement errors resulting from transient spurious sources of heat in the measurement field, sources such as might be provided by heated particles of carbon in the workpiece atmosphere. Although the Kahn patent teaches the use of such herein employed elements as a beamsplitter, dual optical-to-electrical transducers, dual amplifier channels and the use of a workpiece-inherent black body member, the thrust of the Kahn patent is in the direction of performing accurate measurements in the presence of transient spurious sources of heat and in the elimination of effects from these transient spurious heat sources.
The patent of Folke Lofgren et al, U.S. Pat. No. 4,409,476, concerns a fiber optic temperature measurement arrangement which also includes an optical beamsplitter and a pair of optical-to-electrical transducer elements and additionally includes a photoluminescent black body solid material subjected to the temperature being measured. The photoluminescent material used in the Lofgren patent is in the nature of a semiconductor compound and exhibits the characteristic of responding to excitation by a light source, such as a light emitting diode, by emitting light of a different wavelength--the emitted light wavelength being dependent on the temperature of the semiconductor material. The Lofgren patent contemplates use of optical signals of different spectral content at two photodetector devices. An optical filter is inserted in the path of one or both photodetector devices. The Lofgren patent also contemplates the use of time multiplexing in exciting the light emitting diodes--in order to segregate signals resulting from the different photoluminescent detector devices. The Lofgren patent therefore involves a time multiplexed sensing of photoluminescent materials excited by pulses of light emitting diode light using a beamsplitting arrangement for achieving different spectral responses in the detector unit.
The patent of Kenya Goto, U.S. Pat. No. 4,367,040, concerns a multichannel multiplexed optical temperature measuring system wherein light supplied from an external source such as a laser diode, is transmitted bi-directionally along optical fiber transmission paths to and from a reflection or transmission type of optical sensor. The optical sensors of the Goto apparatus modulate the intensity of the supplied light beam in response to temperature or other physical quantities being measured.
The patent of C. E. Everest, U.S. Pat. No. 4,420,265, concerns an infrared responsive temperature measuring system intended primarily for agricultural applications and having the ability to compensate for sky radiation variations included in the measured signal. The patent of G. J. Carlson, U.S. Pat. No. 3,486,378, also concerns a temperature measuring apparatus wherein two detector cells are multiplexed--with the multiplexing of infrared energy in this case being achieved with a rotating disk modulator and with one of the detectors being excited by an incandescent lamp or other reference light source.
While each of these patents help identify the state of the fiber optic, multiplexed signal, temperature meausurement art preceding the present invention, none of the measurement devices taught by these patents achieve the advantages of dynamic range multiplexing and the other techniques of the present invention.